A solid polymer electrolyte is a polymer that is capable of sustaining ionic charge transport. An excellent solid polymer electrolyte is NAFION.RTM., a perflourinated ionomer manufactured by Du Pont. NAFION.RTM., because it is entirely flourinated, is chemically stable and may be exposed to harsh oxidizing and reducing conditions without ill effect. It is known that if electrodes are placed on opposite faces of a NAFION.RTM. membrane, cationic charge (protons) may be "pumped" across the solid polymer electrolyte from one electrode to the other, while each electrode carries out a half-cell reaction. A very desirable feature in this processing scheme is that there be a high surface area of contact between the ionomer matrix of the solid polymer electrolyte and the electrodes. Nevertheless, the electrode cannot cover the surface completely, otherwise the reactant would be blocked from reaching the reactant sites, namely, the electrode-solid polymer electrolyte interface.
Prior art processes for depositing metallic electrodes on the surface of solid polymer electrolytes include mechanically pressing metal particles onto the surface. Patents illustrative of the mechanical approach are U.S. Pat. Nos. 4,272,353; 4,386,987; 4,421,579 and 4,469,579.
According to other prior art practices, noble metals and their alloys have been directly attached to both sides of perfluorosulfonic acid polymer membranes by plating methods utilizing reactions of a metal salt solution with a reducing agent on the membrane surface. According to this method, a reductant, e.g., N.sub.2 H.sub.4, diffuses through the membrane and reduces the noble metal of the salt (e.g., Pt(IV)(CL).sub.6.sup.-4) to produce a platinum coating deposited on one face of the film. This procedure is described in Takenaka, H., et al., "Solid Polymer Electrolyte Water Electrolysis," Int. J. Hydrogen Energy 5, 397-403 (1983).
A method of electrolytic deposition of a metallic electrode on a solid polymer electrolyte is described in U.S. Pat. No. 4,326,930. The two-step method of this patent involves the impregnation of a solid polymer electrolyte with Pt(II) cations by impregnation with a cationic salt, followed by placing this impregnated solid electrolyte material between an anode and cathode to "electrolyze" the metal ions and to deposit a metal layer along the surface of the electrolyte adjacent the cathode. A method for continuously coating a solid polymer electrolyte by this process is disclosed in U.S. Pat. No. 4,396,469.
There is a great deal of interest in solid polymer electrolyte catalytic electrodes because they have utility in fuel cells, sensors, chloralkali processes, dialysis, water electrolysis and, at a more exploratory level, in electroorganic synthesis. Among others, the benefits of solid polymer electrolytic cells are: (1) the polymer serves the dual purpose of electrolyte and separator; (2) mass transfer limitations caused by the limited solubility of gaseous reagents in a liquid electrolyte are avoided; and (3) solid polymer electrolyte cells can be fabricated as a thin film which reduces the weight and ohmic losses.
Despite extensive activity in the production and use of electrodes of the mentioned type, and the many structures and processes proposed, both speculative as well as experimentally evaluated, the need still exists for an effective and commerically viable process for reliably forming such electrodes.